Extreme Heat

Collection Overview

Extreme heat is an increasingly urgent global challenge, with profound implications for health, livelihoods, and ecosystems. The World Health Organization defines a heatwave as “a period where local excess heat accumulates over a sequence of unusually hot days and nights.”1 Due to climate change, heatwaves have become more frequent, more intense, longer in duration, and greater in magnitude across every region of the world. The scale of this acceleration is stark: according to the 2025 Lancet Countdown Report, 84% of the heatwave days experienced annually between 2020 and 2024 would not have occurred without climate change, and heat-related mortality has increased by 63% since the 1990s, reaching a yearly average of 546,000 deaths between 2012 and 2021.2

The same report also highlights that the impacts of extreme heat are not evenly distributed. Certain populations, such as infants younger than 1 year and adults older than 65 years, are especially vulnerable to heat stress and its cascading effects. In 2024, these groups experienced record levels of exposure: adults over 65 years old averaged 20.8 heatwave days per person, and infants averaged 20.5 days. These figures represent dramatic increases of 304% and 389%, respectively, compared with the 1986–2005 baseline, underscoring the growing strain on already vulnerable groups.

Prolonged excess heat triggers a wide range of interconnected challenges. It increases the risk of dehydration, heat exhaustion, cardiovascular strain, and other well-documented health conditions.3 Heat can overburden essential health services and infrastructure,4 strain food and water systems,5 disrupt education systems,6, 7, 8 and reduce labor capacity, particularly in outdoor and high-exposure occupations.9, 10, 11 These effects are felt most acutely in low- and middle-HDI countries, where adaptive resources and protective infrastructure may be limited.

This collection brings together a set of short video and written Insights that explore the many dimensions of extreme heat, including its complex interactions with both human and non-human systems, its far-reaching consequences, and the ways in which its impacts can be mitigated. By connecting scientific findings with practical solutions, the collection aims to deepen understanding of extreme heat as not only an environmental issue, but a defining challenge for societies and systems worldwide.